Adhesives are thermoplastic polymers, frequently in solution, which are applied to a first substrate and after drying are present on the substrate in a tacky form, either directly or else after heat activation. After pressing with a second substrate, which either is uncoated or has likewise been provided with a layer of adhesive beforehand, the two substrates are joined to one another. With certain heat-activated adhesives, the cooling process is accompanied by crystallization, which enhances the properties of the bonded joint.
Since the systems described above comprise thermoplastic polymers, the bonded joint can be softened again by heating, and the substrates can be parted from one another again.
The thermoplastic behavior of the adhesives leads to problems in some cases. For instance, the sensitivity of the substrates may not tolerate a particularly high activation temperature whereas temperatures above the softening point of the adhesive are encountered in the application, and this could result in unintended separation of the substrates during the application.
An example here would be the adhesive bonding of footwear soles. In the case of sports footwear especially, crystallizing, aliphatic polyurethane dispersions are often used. The activation temperature is normally 80° C. Bonded joints produced with such adhesives then subsequently have a heat resistance of around 60° C. The footwear industry, however, demands much higher heat resistances here, since on a fairly frequent basis, the sports footwear is washed in the washing machines at temperatures of 60° C. or more, and delamination of the sole is to be ruled out.
An increase in the heat stability may be achieved, for example, through the use of 2-component (2K) adhesives. 2K adhesives are adhesives wherein the components, (a) adhesive component and (b) crosslinker component, must be stored in separate vessels on account of their reactivity. The state of the art is that the two components are mixed shortly before application and immediately begin to react with one another, generally without additional activation. Polyisocyanates are frequently employed as a crosslinker component. Systems and methods of this kind are described in EP 0 206 059 A, for example.
Disadvantages of this method are:                The cost and effort involved in mixing.        After mixing, the system reacts directly. This results in a limited working time and may entail additional waste if the mixture is not consumed within the restricted working time.        In the case of aqueous systems, it is impossible in some cases to observe the progress of the working time, since the viscosity remains constant and the polyisocyanate is consumed by reaction with the water to form urea particles. This may mean that the heat stability is not increased as planned.        Articles which come into contact with the mixed material (e.g., brushes) must frequently be cleaned and ultimately replaced.        The reactivity of the blended mixtures frequently results in instances of fouling, particularly with non ideal processing conditions, and instances of clogging of the application assemblies, particularly when using rolls, extruders or spraying heads, and hence results in poor-quality adhesive-application outcomes and increased cleaning cost and labor.        
Attempts have already been made to solve at least part of these problems by means of a method wherein the components are mixed in the spraying jet. In terms of their apparatus, however, the construction of these methods is so complex that they are only suitable for use on a laboratory scale. Industrial-scale establishment is hardly possible, or is possible only with very great complexity and high costs, since the precise conditions required for the components to experience sufficient mixing in the spraying jet itself cannot be ensured under ambient conditions and over relatively long sections.